[GHC] #13125: GHC Panics when encoding Russel's paradox using GADTs

[GHC]

#13125: GHC Panics when encoding Russel's paradox using GADTs
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           Reporter:  tysonzero      |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:  panic, GADTs   |  Operating System:  MacOS X
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The following encoding of Russel's paradox using GADTs causes GHC to
 panic, which is understandable as it involves creating a value of an
 uninhabited data type without using any recursion, which should not be
 possible as far as I can tell, so this should probably not type check.

 {{{#!hs
 {-# LANGUAGE GADTs #-}

 data False

 data R a where
     MkR :: (c (c ()) -> False) -> R (c ())

 condFalse :: R (R ()) -> False
 condFalse x@(MkR f) = f x

 absurd :: False
 absurd = condFalse (MkR condFalse)

 main :: IO ()
 main = absurd `seq` print ()
 }}}

 When compiled with optimization I get:

 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.1 for x86_64-apple-darwin):
         Simplifier ticks exhausted
   When trying UnfoldingDone condFalse
   To increase the limit, use -fsimpl-tick-factor=N (default 100)
   If you need to do this, let GHC HQ know, and what factor you needed
   To see detailed counts use -ddump-simpl-stats
   Total ticks: 6441
 }}}

 When I instead load it into GHCi and type "absurd `seq` ()" I get:

 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.1 for x86_64-apple-darwin):
         atomPrimRep case absurd of _ [Occ=Dead] { }
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13125>
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